The present invention relates to an electrodeionization apparatus, a method of operating an electrodeionization apparatus, and a system for producing ultra pure water.
Deionized water is used for various purposes, for example, in plants such as for semiconductor production and liquid crystal display production, in industrial facilities such as for pharmaceutical industry, food industry, and electric power industry, even in households, and in laboratories. Electrodeionization apparatuses are frequently used to produce deionized water as described in Japanese Patent No. 1782943, Japanese Patent No. 2751090, and Japanese Patent No. 2699256. A conventional electrodeionization apparatus of FIG. 2 includes electrodes which consist of an anode 11xe2x80x2 and a cathode 12xe2x80x2, anion-exchange membranes 13 and cation-exchange membranes 14xe2x80x2. The membranes are alternately arranged in such a manner as to alternately form concentrating compartments 15xe2x80x2 and desalting compartments 16xe2x80x2 between the anode and the cathode. The desalting compartments 16xe2x80x2 are filled with anion-exchanger and cation-exchanger made of ion exchange resin, ion exchange fibers, or graft exchanger. In the desalting compartments 16xe2x80x2, the anion-exchanger and cation-exchanger are in the mixed state or multiple-layered state.
Ions flowing into the desalting compartments 16xe2x80x2 react with the ion exchanger according to the affinity, concentration, and mobility of the ions and move through the ion exchanger in a direction of potential gradient. The ions further pass through the membranes to hold neutralization of charges in all of the compartments. The ions decrease in the desalting compartments 16xe2x80x2 and increase in the concentrating compartments 15xe2x80x2 because of the semi-permeability of the membranes and the polarities of potential gradient. This means that cations permeate the cation-exchange membranes 14xe2x80x2 and anions permeate the anion-exchange membranes 13xe2x80x2 so that the cations and anions are concentrated in the concentrating compartments 15xe2x80x2. Therefore, deionized water (pure water) as product water is recovered from the desalting compartments 16xe2x80x2.
Electrode water flows through an anolyte compartment 17xe2x80x2 and a catholyte compartment 18xe2x80x2. The water flowing out of the concentrating compartments 15xe2x80x2 (concentrated water) and having high ion concentration is used as the electrode water in order to ensure the electric conductivity.
Raw water is introduced into the desalting compartments 16xe2x80x2 and the concentrating compartments 15xe2x80x2. Deionized water (pure water) is taken out from the desalting compartments 16xe2x80x2. Concentrated water in which ions are concentrated is discharged from the concentrating compartments 15xe2x80x2. A part of the concentrated water is circulated into the inlets of the concentrating compartments 15xe2x80x2 by a pump (not shown) in order to improve the product water recovery. Another part of the concentrated water is supplied to the inlet of the anolyte compartment 17xe2x80x2. The reminder of the concentrated water is discharged as waste water out of a circulatory system in order to prevent the ion concentration in the circulatory system. Water flowing out of the anolyte compartment 17xe2x80x2 is supplied to the inlet of the catholyte compartment 18xe2x80x2. Water flowing out of the catholyte compartment 18xe2x80x2 is discharged as waste water out of the circulating system.
The pH in the anolyte compartment 17xe2x80x2 is lowered due to H+ generated by dissociation of water. On the other hand, the pH in the catholyte compartment 18xe2x80x2 is increased due to generation of OHxe2x88x92. The acid water flowing out of the anolyte compartment 17xe2x80x2 flows into the catholyte compartment 18xe2x80x2 so that alkalinity in the catholyte compartment 18xe2x80x2 can be neutralized, thereby eliminating damages due to scale formation.
Filling activated carbon or ion-exchange resin into electrode compartments is disclosed in U.S. Pat. No. 5,868,915.
The above conventional electrodeionization apparatus do not remove silica and boron at extremely high ratio.
It is an object of the present invention to provide an electrodeionization apparatus which removes silica and boron at extremely high ratio, a method of operating the same, and a system employing the electrodeionization apparatus for producing ultra pure water.
A method for electrodeionization according to a first aspect of the present invention employs an electrodeionization apparatus which has an anolyte compartment having an anode, a catholyte compartment having a cathode, at least one concentrating compartment, and at least one desalting compartment. The concentrating compartment(s) and the desalting compartment(s) are formed between the anolyte compartment and the catholyte compartment by alternately arranging at least one anion-exchange membrane(s) and at least one cation-exchange membrane(s). The desalting compartment(s) is (are) filled with ion-exchanger, and the concentrating compartment(s) is (are) filled with ion-exchanger, activated carbon, or an electric conductor. Electrode water flows into the anolyte compartment(s) and the catholyte compartment(s). Concentrated water flows into the concentrating compartment(s). Raw water flows into the desalting compartment(s) and the deionized water flows out of the desalting compartment(s). The concentrated water includes silica or boron at a lower concentration than the raw water. The concentrated water flows into the concentrating compartment(s) at a side near an outlet for the deionized water of the desalting compartment and flows out from the concentrating compartment(s) at a side near an inlet for the raw water of the desalting compartment. At least a part of the concentrated water flowing out of the concentrating compartments is discharged out of the circulating system.
An electrodeionization apparatus according to a second aspect of the present invention has an anolyte compartment having an anode, a catholyte compartment having a cathode, at least one concentrating compartment, and at least one desalting compartment. The concentrating compartment(s) and the desalting compartment(s) are alternately formed between the anolyte compartment and the catholyte compartment by alternately arranging at least one anion-exchange membrane(s) and at least one cation-exchange membrane(s). The desalting compartment(s) is (are) filled with ion-exchanger, and the concentrating compartment(s) is (are) filled with ion-exchanger, activated carbon, or an electric conductor. The electrodeionization apparatus further has a device for introducing electrode water into the anolyte compartment and the catholyte compartment; a concentrated water introducing device for introducing concentrated water into the concentrating compartment(s); and a device for introducing raw water into the desalting compartment(s) to produce the deionized water. The concentrated water introducing device introduces water containing silica or boron at a lower concentration than the raw water into the concentrating compartment(s) at a side near an outlet for the deionized water of the desalting compartment(s). The concentrated water flows out of the concentrating compartments at a side near an inlet for the raw water of the desalting compartment(s). At least a part of concentrated water flowing out of the concentrating compartments is discharged out of a circulating system.
A system for producing ultra pure water of the present invention has the above electrodeionization apparatus of the second aspect of the present invention.
Decreasing in silica or boron concentration in the concentrated water flowing into the concentrating compartment near the outlet for the product water leads to decrease in silica or boron concentration in the product water.
By introducing concentrated water containing silica or boron at a lower concentration than the raw water into the desalting compartments at a side near the outlet for the deionized water (product water) in a direction toward a side near the inlet for the raw water, the silica or boron concentration of product water is significantly decreased.
By introducing water having low electric conductivity and high resistivity in the concentrating compartments, the electric resistance of the water in the concentrating compartments is increased.
The ion exchanger such as ion-exchange resin, activated carbon, or electric conductor filled in the concentrating compartments reduces the electric resistance of the concentrating compartments, allowing much electric current to flow.
H+ ions and OHxe2x88x92 ions produced by dissociation of water among the ion-exchange resins carry electric charges, so that voltage between electrodes is prevented from rising so as to allow enough current to flow between the electrodes even when water having high resistivity such as ultra pure water flows in the electrodeionization apparatus.